Band gap (BG) voltage reference circuits provide a fixed voltage reference for integrated circuits. Referring now to FIG. 1, an exemplary BG circuit 10 is shown and includes transistors Q1 and Q2, resistances R1, R2, and R3, a variable resistance Rvar and an amplifier A. Collectors and bases of the transistors Q1 and Q2 are connected to a potential such as ground. The resistance R3 has one end that is connected to an emitter of the transistor Q1 and another end (at potential V1) that is connected to the resistance R1 and an inverting input of the amplifier A. The resistance R1 is connected between one end of the resistance Rvar and one end of the resistance R2. Another end of the resistance R2 (at potential V2) is connected to the emitter of the transistor Q2 and a non-inverting input of the amplifier A. An output of the amplifier A is connected to another end of the resistance Rvar which is at the BG voltage potential Vbg.
Junctions between the emitters and the bases of the transistors Q1 and Q2 operate as diodes. The emitter area of Q1 is typically larger than the emitter area of Q2, where K is a ratio of the emitter area of Q1 divided by the emitter area of Q2. Amplifier A forces the voltage potentials V1=V2. Since the resistances R1=R2, the current flowing into the transistor Q1 is equal to the current flowing into the transistor Q2. Therefore,ΔVbe=|Vbe(Q2)|−|Vbe(Q1)=VT ln(K)Vbg=V(Rvar)+V(R2)+|Vbe(Q2)|
ΔVbe is applied across the resistance R3 to establish a proportional to absolute temperature (PTAT) voltage. The voltages V(Rvar) and V(R2) have positive temperature coefficients. |Vbe(Q3)| has a negative temperature coefficient. Therefore, Vbg has a net temperature coefficient of approximately zero. The resistor Rvar is adjusted to change Vbg and its temperature coefficient.
The accuracy of Vbg is related to the emitter area ratio K and the emitter area. Generally as the emitter area and the emitter area ratio K increases, the accuracy of the BG circuit also increases. As used herein, the term accuracy is used to reflect the variations that occur due to process. Higher accuracy refers to increasing invariance to process. Lower accuracy refers to increasing variance to process.
While increasing accuracy, the power dissipation of the transistor also increases with the area of the emitter. Therefore, the increased precision of the BG circuit is accompanied by an increase in power dissipation. Therefore, circuit designers must tradeoff accuracy and power dissipation.